With ever increasing concern about environmental pollution, electric vehicles (EVs) have recently been receiving more and more attention. The battery system is a critical component of an EV, affecting its performance and safety. The battery system of an EV typically includes two major parts—the batteries and the battery management system. Typical EV battery packs have voltage and capacity exceeding 50V and 400 Wh respectively. Battery cells using lead-acid, nickel metal hydride and lithium ion chemistries have all been used in the past. Among them, lithium ion cells have been highly valued for their high specific energy density and large cycle life. However, lithium ion cells are known to degrade if operated outside of a certain voltage and temperature range. In extreme situations, this degradation may lead to a safety hazard, e.g. fire or even explosion. The high voltage and capacity of an EV battery pack are typically achieved by connecting many batteries in series, wherein each battery may comprise a plurality of battery cells. In such designs the discharge capacity of a battery pack is only as large as the battery member that has the lowest capacity, thus it is important to keep all the battery members balanced in their capacities. To ensure a long and safe operating life in an EV, a lithium ion battery pack requires a management system that can monitor in real time the voltage and temperature of the various cell in the battery pack, keep the cells operating in a predetermined range, and also keep their capacities balanced.
In order to effectively manage a large number of battery or cell members in an EV battery pack, the management system often employs a two-level control comprising a master controller and a plurality of monitors. Each monitor gathers temperature and voltage information for one or more battery elements. The master controller analyzes the information supplied by the monitors and sends commands back to the monitors for execution.
Two typical designs for the communication infrastructure between the master controller and the monitors are illustrated in FIGS. 1 and 2. In both cases, each monitor controls two batteries as an example although other arrangements are possible as will now be apparent to those of ordinary skill in the art.
FIG. 1 shows a typical battery monitoring system 100 employed in electric vehicles. There is a master controller 102 which receives battery voltage and temperatures information over electrical wires 104a, 104b from N monitors M1, M2, . . . , MN-1, MN which, in turn, monitor corresponding batteries 1 and 2, 3 and 4, . . . , 2N-1 and 2N. Master controller 102 communicates corresponding information over lines 106a, 106b to a vehicle controller 108 and also may receives instructions over the same lines from vehicle controller 108. Batteries 1, 2, . . . , 2N are coupled in series in this illustrative embodiment using conductors 110a, 110b to form a relatively high-voltage DC source as shown. Each monitor in this approach is coupled to sense the potential of a pair of batteries with sense lines 112a, 112b. Each monitor in this approach is also coupled to a pair of temperature sensors 114a, 114b which provide the monitor with information indicative of the temperature of the pair of batteries. This design is simple in principle because each monitor has an assigned address (since each is hard-wired back to the master controller 102) with which to report and receive information. But as the number of monitors increases, the wiring of the system becomes complex and can become subject to or cause signal interference. The inflexibility of the system design can also be a disadvantage with particularly large battery packs.
FIG. 2 illustrates another typical battery monitoring system 200. This second design makes use of a data communications bus 202 to which the monitors are connected, as shown in FIG. 2. This structure has the advantages of simplifying synchronized sampling, standardizing connectors, simplifying wiring and providing design flexibility. However the monitor addresses in this approach must be pre-assigned and that complicates component replacement.
In the designs illustrated in FIGS. 1 and 2, metal wires are necessary for providing communication between the monitors and the master controller. This creates at least two problems. First, the battery management system is susceptible to electrical signal interference resulting from the high voltage/high current lines 110a, 110b of the battery pack. Frequently, optoisolation of the monitors is required to alleviate this problem. Second, the large number and wide-spread use of metal wires in the battery pack carries the potential to cause a short circuit in the battery pack through those wires under certain circumstances. With the high-power capability of many EV batteries, such a short circuit could cause serious hazards during or after an accident. Potentially, long-term wear and tear of wire insulation jackets may also lead to safety issues.
A battery pack for an EV (or for other uses) may be balanced by locally applying (or removing) a load for a period of time in order to affect a state of charge of a particular battery or battery cell. One or more of the following example techniques may be used:    a) activating one or more load resistors coupled to a particular battery or battery cell of the battery pack (and ultimately to ground) in response to a command from the master controller 102 in order to reduce the state of charge of the particular battery or battery cell.    b) activating a diagnostic indicator (such as a light-emitting diode (LED)) coupled to a particular battery or battery cell of the battery pack in response to a command from the master controller 102 in order to reduce the state of charge of the particular battery or battery cell.    c) inhibiting a power-saving sleep mode of a monitor coupled to a particular battery or battery cell of the battery pack in response to a command from the master controller 102 in order to reduce the state of charge of the particular battery or battery cell.    d) increasing the power consumption used by the monitor—master controller communication system (as by causing the transmission or receipt of otherwise unnecessary messages) at a monitor coupled to a particular battery or battery cell of the battery pack in response to a command from the master controller 102 in order to reduce the state of charge of the particular battery or battery cell.